Well, it wasn’t. This is pretty well documented. It has pretty much the same design requirements and even some of the same contractors, but it also differed in key ways. It had a totally different instrumentation board, optical packages, and was built for servicing which KH wasn’t.

The fact that it was the same form factor is not an accident, as Hubble was sized to fit in the shuttle payload bay, which was itself sized based on keyhole.